1. Field of the Invention
The bridge terminal output unit operates with a Network Element (NE) used to bridge communications between other NE's that incorporate alarm-based reconfiguration mechanisms and more specifically this invention employs loss-of-signal (LOS) detection to substitute a second bit stream for a time-delayed original bit stream onto an output line to a downstream network terminal.
2. Description of the Related Art
The commercial demand for higher aggregate data rates has led to rapid deployment of new communications technology. Dense Wavelength Division Multiplexed (DWDM) has emerged as the next step in the evolution of high-speed networking. DWDM networks allow the multiplexing of 80 or more 10 GBit/sec data streams onto a single optical fiber. Manufacturers have adopted common standards for channel frequencies and control channel carrier format, and support interfaces to transport most high-speed physical layer protocols including synchronous optical network/synchronous digital hierarchy (SONET/SDH) networks, plesichronous digital hierarchy (PDH) networks, Gigabit Ethernet, and FDDI-II. An example in the prior art in which DWDM based network is used as a network bridge for the transport of a SONET signal is shown in FIG. 1.
DWDM network deployment has been made practical by several new technologies including optical signal multiplexers, optical signal demultiplexers, optical amplifiers, and optical cross-connects. As implemented on a DWDM network terminal, optical signal multiplexers accept multiple optical signals transmitted on different wavelengths, i.e., channels, of light over separate fibers and combine those signals onto a single fiber. DWDM network optical signal demultiplexers accept multiple optical signals (channels) carried on different wavelengths of light over a single fiber and separates the multiple signals onto separate fibers. Optical amplifiers boost the power levels of the multiplexed channels simultaneously, extending the transmission range of DWDM terminals to a practical and useful distance. Optical cross-connects dynamically re-provision DWDM channels. Currently, most DWDM networks are used to consolidate and bridge communications between SONET/SDH, PDH, and other legacy networks.
SONET and SDH constitute the majority of deployed single-channel, wide-area optical communications networks. SONET and SDH provide open physical layer transmission standards for manufacturers of optical network transmission equipment. SONET/SDH defines standard optical signals; a synchronous frame structure for multiplexed digital traffic; bit timing and network element synchronization; alarm thresholds and error recovery; and standard operations, administration, maintenance, and provisioning. DWDM terminals interface with most high-speed communications terminals, such as SONET/SDH and PDH, via optical line input-output units (OLIU). The input to the DWDM terminal is a functional unit that accepts an incoming single-channel optical data stream from the network being bridged, converts it to an electrical binary data stream, retransmits that binary stream as an optical signal at a specific wavelength suitable for optical multiplexing, and presents that signal to the optical multiplexing unit. The output of the DWDM terminal is a functional unit that performs the reverse operation: it accepts an individual wavelength from the multiplexed optical stream, converts the electrical stream to an optical stream suitable for reception by the single channel network being bridged.
Off-the-shelf DWDM terminals are available that interface with some high-speed communications terminals using all-optical inputs and outputs. In this case, the communications terminal of the network being bridged is responsible for generating an optical signal at the proper wavelength and line width suitable for optical multiplexing. The input to the DWDM terminal is a functional unit that accepts an incoming single-channel optical data stream and delivers it directly to the optical multiplexing unit. The output of the DWDM terminal is a functional unit that performs the reverse operation: it accepts an individual wavelength from the multiplexed optical stream, and presents it directly to the connected high-speed communications terminal.
Present DWDM terminals and cross-connects do not operate in a maimer that is transparent to the automatic protection switch (APS) mechanisms of the network being bridged. The APS mechanisms of the networks being bridged, such as SONET/SDH and PDH terminals, must be disabled before DWDM-layer APS or cross-connects occur. If the line between the DWDM-layer terminal and the network being bridged should fail, the bridged network will be unable to automatically restore service. This loss of APS functionality in the bridged network serves to degrade overall network performance.